The present invention relates to a liquid crystal display device containing an X-Y matrix type liquid crystal display panel.
Conventionally, an X-Y matrix type liquid crystal display panel is driven in either of the following two well-known methods: Method A in which the polarity of applied voltage is reversed in one horizontal scanning period as shown in FIG. 6, and Method B in which the polarity of applied voltage is reversed for each frame as shown in FIG. 7. The waveforms shown in FIGS. 6 and 7 include waveform distortion caused by the electrostatic capacity of the liquid crystal panel and by the resistance of the transparent electrodes. Method A provides a smaller ratio of waveform frequency variation in the display pattern than method B does (the frequency change ratio is 2 in method A whereas it is N in the method B when the duty ratio is N), but provides a higher frequency in general, resulting in larger power consumption. When using a larger liquid crystal display panel in which the liquid crystal capacity and the electrode resistance increase, method A is influenced significantly by waveform distortion so that the effective applied voltage drops. Because of this reason, method A is hardly used for large liquid crystal display panels.
Presently, therefore, an X-Y matrix type liquid crystal display panel is driven by method B. For a large high density liquid crystal panel in which the number of time divisions exceeds 100, however, method B tends to cause irregular picture and crosstalk which deteriorates the picture quality seriously.
FIG. 8 shows the typical crosstalk phenomenon. A pattern is shown where black portions 2 should be normally displayed against a white background 1 suffers crosstalk so that portion 3 which should be white become gray. The driving waveforms for the portions 1 and 3 are shown in FIG. 9; waveforms (1) and (2), respectively. In the waveform (1) of FIG. 9 that is, of the portion 1 in FIG. 8, the driving frequency component of the display pattern is mainly a low frequency, whereas in the waveform (2) of FIG. 9, that is, in the portions 3 of FIG. 8, the driving frequency component of the display pattern is mainly a high frequency. The difference in the frequency component of the driving waveforms results in a conspicuous crosstalk phenomenon. In other words, crosstalk can be caused by the diversified frequency characteristic of the threshold voltage of the liquid crystal display panel or by the variation of effective voltage caused by distorted driving waveform.
The former cause occurs when the threshold voltage of the liquid crystal display panel changes in a driving frequency band although the effective voltage is constant. The driving frequency band varies depending upon the driving method. As mentioned above, the frequency variation ratios of the conventional methods A and B are 2 and N (N is a duty ratio), respectively. When the threshold voltage of the liquid crystal display panel changes with frequency, method A is advantageous over method B in terms of the crosstalk phenomenon because the driving frequency variation ratio is smaller in method A. On the other hand, method A has a disadvantage of larger power consumption.
A driving method from which the above problems are eliminated has been proposed. This method is to reverse the polarity of driving voltage applied to the liquid crystal display panel at intervals corresponding to specified horizontal scanning periods. According to this method, the advantage of method B can be made use of, while power consumption is minimized. To explain this method, the driving waveforms in which the polarity of the waveforms (1) and (2) of FIG. 9 is reversed every four horizontal scanning periods (4H) are shown in waveforms (1) and (2) of FIG. 10, respectively. In these waveforms, the frequency of polarity reversing signal is the major component of the driving frequency, so that the influence by the frequency of the display pattern is reduced. Namely, in this method, the driving frequency having a low frequency component near the frame frequency is shifted to the higher frequency side so as to equalize the driving frequency component for each picture element. Moreover, the waveform distortion is also equalized as shown in FIG. 10, and the effective voltage value is held constant to some extent in this method.
The above method has an effect of reducing crosstalk phenomenon. But it has another problem in that linear display irregularity is generated along the scanning lines when polarity is reversed. This display irregularity is caused by the following reason.
In waveforms (1)-(5) of FIG. 11 show examples of driving waveforms in the liquid crystal display device. In these figures, waveform distortion caused by the electrostatic capacity of the liquid crystal panel and by the resistance of the transparent electrodes is also taken into account.
Waveforms (1) and (2) of FIG. 11 show the waveform of the driving voltage applied to the scanning electrodes. The waveform (1) of FIG. 11 is for the case where a selection pulse is generated immediately after the reversal of polarity, and the waveform (2) of FIG. 11 is for another case. Waveform (3) of FIG. 11 shows the waveform of driving voltage applied to the signal electrodes. This waveform is for the case where all picture elements are turned off. Waveform (4) of FIG. 11 shows the potential difference between the waveform (1) of FIG. 11 and that of waveform (3), and waveform (5) of FIG. 11 shows the potential difference between the waveforms of (2) and (3) of FIG. 11. Both are the waveforms of the voltage applied to the picture elements. As shown, waveform distortion is different between waveforms (4) and (5) of FIG. 11. This difference in the waveform distortion causes a uniform effective voltage to be applied to picture elements, resulting in the linear display irregularity. This problem can be solved by shifting the polarity reversing point by 1H (one horizontal scanning period) in each frame to equalize the waveform distortion in each scanning line, thereby making the effective voltage uniform. In this case, however, a driving frequency component smaller than the frame frequency is produced. This results in meandering display irregularity which occurs in the downward direction on the screen during the sequential scanning.
As mentioned above, crosstalk occurs in the conventional liquid crystal display device, and if action is taken to eliminate the crosstalk, linear display irregularity or meandering phenomenon is observed on the screen.